Heat treated uranium alloy and method of preparing same



D. w. WHITE, JR

3 She ets-Sheet 1 450 n g Y e 6% 400 E 9/0 2 o g o a m 96/ g 350 u 7 I00xdm;

" 05 l 5 l0 50 I00 500 I000 rmz-muurss INVENTOR. DONALD W. WHlTE,Jr.

ATTORNEY Aprii 16, 1957 D. w. WHITE, JR

.HEAT TREATED URANIUM ALLOY AND METHOD OF PREPARING SAME Filed D80. 22,1952 3 SheetS-Shqet 2 5 TIME-MINUTES INVENTOR.

DONALD w. WHlTE,Jr. BY

ATTORNEY HEAT TREATED URANIUM ALLOY AND METHOD OF PREPARING SAME DonaldW. White, Jr., Burnt Hills, N. Y., assignor to the United States ofAmerica as represented by the United States Atomic Energy CommissionApplication December 22, 1952, Serial No. 327,248

6 Claims. (Cl. 148-13) This invention deals with a process ofdimensionally stabilizing uranium-base metals, and in particularuranium-chromium alloys subjected to nuclear irradiation, and with theproducts obtained by the process.

It is known that surface and dimensional stability of uranium materialssubjected to nuclear irradiation are dependent on a fine and uniformgrain size arbitrarily arranged without preferred orientation. A finegrain size has been obtained heretofore in uranium by working the metalat so-called alpha-phase temperatures, that is, at temperatures up toabout 660 C. However, this uranium material is highly oriented and isdimensionally unstable when subjected to nuclear irradiation. Whenuranium is heated to temperatures at which it exists in the so-calledbeta-phase, that is, between 660 and 770 C., and is thereafter quenchedto an alpha-phase temperature, a material having a relatively randomgrain orientation with improved dimensional stability is obtained. Thisprocess forms the subject matter of copending application Serial No.235,115, filed by Raymond Ward and Alden B. Grenginger on July 3, 1951.However, the grains are relatively large and as a result some surfaceroughening of the uranium is encountered upon nuclear irradiation.

A further improvement has been obtained by alloying the uranium with asmall amount of chromium metal and then beta-heating and alpha-quenchingthe alloy. This latter improvement forms the subject matter of copendingapplication Serial No. 314,734, filed on October 14, 1952, by John R.Keeler and Raymond Ward.

It is an object of the present invention to obtain further improvementsin the surface and dimensional stability of uranium-base materials whensubjected to neutron irradiation as for example, when used in neutronicreactors such as power piles.

It is another object of this invention to provide a uranium metal whichcontains crystals of a very small uniform size arranged in anon-oriental distribution.

It is still another object of this invention to provide a uranium metalwhich retains its smooth surface even after exposure to neutronirradiation.

It is furthermore an object of this invention to provide a uranium metalwhich is comparatively hard.

It is also an object of this invention to provide a uranium alloypossessing the combination of excellent surface and dimensionalstability, only slight dilution of the uranium and a relatively lowneutron capture cross section.

It is finally also an object of this invention to provide a uranium-basemetal that has a high degree of dimensional stability and a smoothsurface and retains these properties after repeated thermocycling.

These and other objects are accomplished by alloying the uranium withfrom 0.1 to 10 atomic percent of chromium metal, heating the alloyobtained at least to betaphase temperatures, quenching the alloy to aselected temperature of the alpha-phase range, this selected temperatureranging in general from about 400 C. to 525 C.,' and holding it at thistemperature until transformaatent I another test.

tion to alpha-phase crystals is complete. However, on alloys containingless than 0.6 atomic percent chromium, the minimum temperature to whichthe alloys must be quenched is higher than 400 C.

A number of uranium-chromium alloys were prepared for investigating theeffect of holding temperatures in the alpha-phase or isothermaltransformation on the physical and mechanical characteristics. Pureuranium metal and electrolytic chromium were used for this purpose invarying proportions; the metals were vacuummelted in zircon cruciblesand cast into graphite molds. The alloys obtained, after annealing at550 C., were swaged in the form of rods l inch in diameter. During theswaging process the alloys were annealed at 550 C. after every percentreduction in area. The rods were cut into specimens of equal length,annealed between 700 and 720 C. for several hours and thenfurnacecooled.

The specimens, after being coated with a protective film of siliconegrease, were converted to beta-phase condition by immersion in atin-lead bath of a temperature above 630 C., specifically of about 715C., for 15 minutes. Thereafter the specimens were cooled down toalpha-phase temperature by immersion in another tin-lead bath ofcorresponding alpha-phase temperature.

The transformation of uranium from beta-phase or the gamma-phase to thealpha-phase condition is always accompanied by contraction. Thisphenomenon was utilized to ascertain the progress of thistransformation; measurements of the contraction were carried out inregular intervals with a dilatometer, and the specimens were held in thealpha-temperature bath until the dilatometer measurements indicated thatno further contraction was taking place.

Four diiterent alloys were prepared and used for the investigation whichled to this invention, namely alloys containing 0.3, 0.6, 1.8 and 4atomic percent of chromium, respectively. The temperature selected forisothermal transformation treatment ranged between and.

630 C. The periods of time required for the begining and completion ofphase transformation at the various temperatures were determined. Thefindings were plotted as diagrams; they are shown in the accompanyingdrawings, Figures 1-4. In these diagrams, the temperatures used for theisothermal transformation are plotted on the ordinate, and the periodsof time necessary for the beginning of alpha-phase formation (indicatedby crosses) and those necessary for complete transformation (indicatedby circles) are entered on the logarithmic-scale abscissa.

As previously mentioned, the specimens were directly quenched from thebeta-phase to the alpha-phase temperature selected for isothermaltransformation; however, for the alloy containing 0.3 atomic percent ofchromium and the isothermal temperature of 100 C., uenching in ice waterand reheating in boiling water were found necessary, because otherwisesome transformation would take place in the transition throughintermediate temperature ranges and thus give misleading results. Twocontrol experiments indicated that this procedure did not bring abouterroneous findings. These control experiments were carried out with analloy containing 0.6 atomic percent of chromium, one set at 100 and oneat 200 C.; in each instance, quenching was carried out to thetemperature of isothermal treatment, directly in one test an d with icewater followed by reheating in In both instances identical results wereobtained when quenching was carried out directly and when it waseffected via the intermediate step of cooling in ice water. V I

The curves shown in the drawings are single-C- shaped for the alloyshaving chromium contents of 1.8

and 4 atomic percent and double-C-shaped for those having 0.3 and 0.6atomic percent of chromium. Considering the upper Cs of the double-6curves, it will be obvious that the maximum rate, of transformation inall four alloys is at about 570 C., while the maximum rate oftransformation in the two lower-C sections is located between 250 and300 C. This lower maximum rate is faster than that of the upper Cs;however, it was found that the grain size, when the isothermal treatmentwas carried out within the range of the lower US, was very irregular,while a uniform and finer grain size was obtained when treatment wascarried out in the lower portion of the upper Cs. Since the dimensionalstability is closely related to, and dependent upon, the grain size,treatment in the lower portion of the upper range is required.

Actually, while the curves in Figures 3 and 4- are single-c shaped andthose in Figures 1 and 2 are double-C-shaped, the single Cs in factcorrespond to the upper Us in the double -shaped curves insofar as thepractice of the present invention is concerned. In other words, in thosecases where only a single C is shown, it is believed there actuallyexists a lower 0 curve at lower temperatures than those employed in theparticular experiments.

In the following Table I the grain diameters of a uranium alloycontaining 0.6 atomic percent of chromium are given as they wereobtained by isothermal transformation treatment at various temperaturesshown.

Table I Temperature of isothermal Position on diagram, transformation,O. Fig. 2

Upper C do....

This table indicates that the smallest grain sizes are obtained when thetemperature of isothermal transformation is in the upper 0 up toslightly above 515 C. and that larger grains are obtained at highertemperatures as well as at all temperatures in the region of the lower0. Optimum transformation temperature range for the 0.6 atomicpercent-alloy is from about 400 C., i. e., the transition temperaturebetween the upper and lower 0, and a maximum temperature of about 525 C.The upper limit of 525 C. holds for all of the alloys, but the lowerlimit varies with the chromium content and broadly may be defined as thetransition temperature between the two Cs. However, as longtimes-at-temperature are required to obtain a complete transformation ofalloys having a high chromium content even when operating well above thetransition temperatures, for practical purposes the isothermaltransformation treatment will usually be carried out at temperatures ofat least 400 C.

i The chromium content itself was found also to have a slight bearing onthe grain size. For instance, after isothermal treatment at 515 C., arelationship between chromium content and grain size was determined asshown in Table II. These data show that the grain size slightlydecreases with increasing chromium content.

1.8 at. percent Cr 0.0 35 0.03

-In order to determine whether the above-described improvements areactually due to the isothermal treat- 4 at. percent Cr ment, theuranium-chromium alloys were beta-heated as described above, but then,instead of holding them at the alpha-temperature until completetransformation, they were merely quenched to room temperature. It wasfound that the optimum properties achieved by the isothermal treatmentof this invention were not obtained by beta-heating and quenching alone.Moreover, the alloys having higher chromium contents (above 1.8 atomicpercent) showed a tendency to crack when thermally cycled.

The hardness of the isothermally treated alloys was also found to beimproved by the isothermal treatment, the lowest temperatures of theupper C's (and of the single Cs) yielding the highest hardness values.The hardness was also found to increase with increasing chromiumcontent.

A convenient laboratory means for predicting dimensional stability of auranium sample under irradiation is a thermal cycling test, in which thesample is repeatedly heated and cooled. As a rule a uranium materialwhich exhibits dimensional or surface instability during this test willbe similarly unstable under irradiation. Therefore, in order todetermine the dimensional stability of some of the alloys treated by theprocess of this invention, isothermally treated specimens were sealed inPyrex tubes in which a partial pressure of helium prevailed, and thespecimens were then thermally cycled. The results are compiled in TableIII.

Table III Temp. of Dimensional Sample Iso- Change, Percent NumberMaterial thermal Surface Treatment, C. Diarn. Length percent CL... 449+0. 13 +0. 06 A1 percent Cr.... 449 +0. 13 +0.10 A1 percent On... 489+0.10 +0. 02 A2 percent CL... 489 +0. 08 0. 08 A2 percent Gr-.. 513+0.05 0 A3 percent Cr.... 513 +0. 10 0 A3 percent Or 547 +0. 03 +0. 02A4 1 .percent Or 547 +0.05 02 A4 percent Or 581 +0. 03 0. 02 A5 1percent CL.-. 581 0. ()8 +0.02 A5 percent CL... 614 0. 28 0. 34 Cpercent 01".... 614. -0. 35 0. 40 C percent Cr.. +0. 48 +0.20 B4 percentCL.-. 265 +0. 83 0. 07 B3 percent CI.... 377 Specimen oxidized in testpercent Cr.... 3% +0. 13 +0.20 B1 percent Or 452 +0. 28 +0. 60 B1percent Cr.... 515 +0.08 +0. 91 B2 percent Gr.... 574 +0.20 +0. 49 B7percent Cr.. 624 +0.58 +1. (34 C4 percent 01".... 100 +1.05 +0.20 G3percent Gr.... 265 +0. 94 +0. 24 C2 percent Cr... 376 Specimen oxidizedin percent CL... 452 +0. 70 +0. 38 C1 percent Cr.... 480 +0. 48 +0. 49B6 percent Cr-... 515 0. 13 +0. 91 B3 percent Gr.... 574 +0. 40 +0. 47B5 percent Or.... 624 +0.88 +1. 66 05 While 500 cycles between 100 and500 C. were used for samples No. l-l2, samples No. 13-28 were subjectedto 554 cycles between 100 and 525 C.

It will be noted that the dimensional changes of the samples prepared inaccordance with the present invention are extremely low as compared withknown uranium products. For example, when subjected to the thermalcycling test employed, a fine-grain uranium obtained by working themetal at so-called alpha-phase temperatures, that is, at temperatures upto about 660 C., will usually exhibit a change in length of about 15 to20 percent while a uranium sample which has been heated to a beta-phasetemperature of from 660 to 770 C. and then quenched to an alpha-phasetemperature will change in length about l-2 percent and will alsoexhibit some surface roughening or blistering.

Table III, in addition to the dimensional changes, lists the surfaceconditions of the isothermally treated specimens in the last column, Aindicating a very smooth, virtually unchanged surface, B a smoothsurface, and C a rough surface comparable to that of the best unalloyeduranium. Minor variations within the A, B and C groups are shown byadded numbers, 1 being the smoothest, 2 the next smoothest surface, andso on.

Table HI shows that with temperatures between 400 and 525 C., the finestgrain size and the greatest dimensional plus surface stability wereobtained.

The foregoing data also show that such uranium alloy-s can be used asstructural members in devices, other than reactors, where the devicesare subjected to such fluctuations of temperature and where unalloyeduranium would be unsuitable due to a greater change in dimensions andsurfaces.

As has been mentioned in the beginning, the process of this invention isapplicable to alloys containing from 1.0 to 10 atomic percent ofchromium; it is especially well suitable for alloys containing from 0.3to 4 atomic percent of chromium.

It will be understood that this invention is not to be limited to thedetails given herein but that it may be modified within the scope of theappended claims.

What is claimed is:

1. A process of producing small grain size uranium of great dimensionaland surface stability, comprising incorporating from 0.1 to 10 atomicpercent of chromium into the uranium, heating the alloy obtained to anelevated temperature above the alpha-to-beta transformation temperature,rapidly cooling the alloy to a temperature of between about 525 and 400C., and holding the alloy at this latter temperature untiltransformation to alpha-phase crystals is complete.

2. The process of claim 1 wherein the alloy contains from 0.3 to 4atomic percent of chromium.

3. The process of claim 2 wherein the elevated temperature is between660 and 770 C. and is maintained for 15 minutes.

4. The process of claim 3 wherein the elevated temperature is about 715C.

5. A composition of matter obtained by the process of claim 1.

6. A composition of matter obtained by the process of claim 2.

References Cited in the file of this patent UNITED STATES PATENTS MorrisJuly 31, 1956 OTHER REFERENCES Metallic Uranium, 14 pages, declassifiedSeptember 23,

(Copy in Patent Office Library.)

1. A PROCESS OF PRODUCING SMALL GRAIN SIZE URANIUM OF GREAT DIMENSIONALAND SURFACE STABILITY, COMPRISING INCORPORATING FROM 0.1 TO 10 ATOMICPERCENT OF CHROMIUM INTO THE URANIUM, HEATING THE ALLOY OBTAINED TO ANELEVATED TEMPERATURE ABOVE THE ALPHA-TO-BETA TRANSFORMATION TEMPERATURE,RAPIDLY COOLING THE ALLOY TO A TEMPERTURE OF BETWEEN ABOUT 525 AND400*C., AND HOLDING THE ALLOY AT THIS LATTER TEMPERATURE UNTILTRANSFORMATION TO ALPHA-PHASE CRYSTALS IS COMPLETE.